How do I top up my car’s brake fluid?

brake fluid

Other than pushing on the middle pedal, we don’t have much to do with our car’s brakes. But you should keep an eye on the reservoir of brake fluid and if it needs it, top it up.

Where is the brake fluid reservoir?

You will find the fluid reservoir beneath the bonnet. It is usually in a white plastic container with a black lid. This may well have a pictogram of a brake disc and pads on it. Inside you should be able to see a liquid. This is brake fluid. If you’re unsure which is brake fluid and which is coolant, consult your car’s user manual.

How to check it

First of all, take the car for a short drive making sure you use the brakes a few times. This enables the fluid to be distributed properly around the system. Then park the car on a flat surface; pick somewhere to stop that enables you to move around the car and look under the bonnet safely.

It’s pointless checking the brake fluid when the car is parked on a slope: you won’t get an accurate reading of how much liquid you have.

The level of liquid in the brake reservoir should be somewhere between the ‘minimum’ and ‘maximum’ markers.

How to fill the reservoir

First wipe the lid. This is so that if there’s any oily dust on it, there’s no danger of any tiny particles falling into the fluid reservoir where they might cause a blockage. Then unscrew the lid. If the level of the liquid is towards or below the ‘minimum’ marker, add liquid until it’s at the maximum level.

Replace the lid then go for a drive. Park up on a flat surface again and check the level. It should still be where you filled it up to. Brake fluid is corrosive so take care to wipe it away if you spill it on paintwork or in the engine bay.

What is brake fluid?

When you press the middle pedal in your car, you couldn’t hope to exert sufficient pressure for the calipers to grip the discs without a little hydraulic help. This is where brake fluid comes in. When you push the brake pedal, it applies hydraulic pressure to help the calipers grab the discs.

Theoretically you could use water but this freezes and has a relatively low boiling point (under heavy braking, the fluid in brake systems compresses and can get very hot). By using a silicone-based liquid, brake fluids can retain the same viscosity (their liquidity) across a wide range of temperatures. This helps give your car’s brakes a constant feel and performance.

brake fluid

When should you change brake fluid?

Different car makers offer different recommendations over changing fluid. Some say every two or three years, others every five or six. Check your user manual. This should also say what type of fluid your car needs. Broadly, if the fluid in your car looks murky or very dark, you should consider having it changed. The same goes if it has bits of debris in it.

Brake fluid can degrade over time. This happens when it absorbs water that can get into the brake system. The water then boils off when it gets hot, creating air locks in the system and reducing your braking power. Garages should have a brake fluid testing kit (main picture).

Braking systems are sealed units. If the fluid level is lower than it should be, there’s probably a leak somewhere. Get a technician to check it out.